Except for oxygen,
all gases are asphyxiants. Even though a gas is completely nontoxic, it
can easily cause suffocation unless it is mixed with sufficient oxygen
to support life. Nitrogen, for example, is a harmless, inert gas which
makes up about 78% of the air we normally breath. However, only a few
breaths of pure nitrogen could render a person unconscious because nitrogen
alone will not support life.